Self-supporting sheet-like structure comprising a substrate and a coating, and a process for its production

ABSTRACT

A self-supporting sheet-like structure comprises a coating, which reduces sliding friction, on at least one surface of the substrate. The slip-improving coating is produced by treating this substrate surface by means of an electric corona discharge between a high voltage electrode and a grounded counter-electrode while simultaneously spraying an aerosol into the corona discharge zone. The aerosol contains a film-forming agent as a slip agent, for example aqueous solutions or emulsions of monoesters of glycerol and long-chain fatty acids.

This is a division of application Ser. No. 07/391,076 filed Aug. 8,1989.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a self-supporting sheet-like structurecontaining a substrate layer and a coating on at least one surface ofthe substrate layer. The coating composition includes a film-formingagent.

The invention furthermore relates to a process for the production of asheet-like structure according to the invention, having a coating. Thecoating is applied as an aerosol simultaneously with a corona treatmentto the sheet-like structure.

2) Prior Art

In the case of sheet-like moldings of polymeric materials, such as filmsor sheets, inadequate slip and frictional properties during productionand further processing of the said materials lead to many undesirableproblems. Static friction and sliding friction and blocking against oneanother and against guide or direction-changing elements, such as, forexample, rollers, are examples of some of the undesirable problems.

A conventional suitable solution for solving this problem consists ofincorporating in the material of the substrate layer inert particleswhich lead to a certain surface structure and thus have an advantageouseffect on the sliding behavior of the molding. Examples of such inertparticles, which can be incorporated either during the preparation ofthe thermoplastic or via a masterbatch during production of the molding,are inorganic natural or synthetic compounds, such as BaSO₄, CaCO₃,TiO₂, kaolin, Ca phosphates, etc., or organic particles, for examplethose consisting of crosslinked acrylates. The concentrations varywithin wide limits, i.e., in the range from 0.001 to 1.0% by weight.

As already mentioned, a substantial improvement in the slidingproperties of the moldings can be achieved by this solution, which alsohas an advantageous effect on the performance characteristics of thearticles produced therefrom.

However, the disadvantage here is that the inert particles, beforeincorporation, have to be subjected to a very complicated, expensivetreatment process in order to avoid other disadvantages, such as, forexample, increased abrasion on contact with guide elements and the like.Furthermore, the optical properties of the substrates can also beadversely affected in some cases.

EP-A-O 153 853 describes a substrate film for a magnetic tape, the saidfilm having a net-like coating which contains fine particles and impartsthe necessary frictional properties to the said substrate film. In thisprocess, the coating materials, such as the polymeric binders containingcolloidal silicates, have to be mixed together and the mixture appliedto the surface of the substrate between the individual orientationsteps. One of the disadvantages of this process is that the formation ofthe structures necessitates at least one stretching step, i.e., islinked with the process for the production of the film.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide sheet-likestructures, such as films, sheets and the like, which are easy to handleand have slip-promoting surfaces, where the slip-promoting substancesand the structures resulting from these can be applied independently ofthe production process and are still permanently and firmly anchored tothe substrate.

This object is achieved by a sheet-like structure of the type stated atthe outset, whose defining feature is that a coating is produced bytreating at least one surface of the substrate layer by means of anelectric corona discharge which takes place between high voltageelectrodes and a grounded counter-electrode, with an aerosol containinga polymeric film-forming agent simultaneously being introduced into thecorona discharge space during the corona discharge.

BRIEF DESCRIPTION OF THE DRAWING

The FIG. 1 of the drawing is a part schematic view, part cross-sectionalview of the aerosol/corona treatment apparatus of the present invention.

FIG. 2 is a scanning electron micrograph of the surface of a film of thepresent invention under 300 times magnification.

FIG. 3 is a scanning electron micrograph of the surface of a film of thepresent invention under 300 times magnification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of the present invention, it is possible in principleto use, for the substrate layer, all materials which have thedifficulties stated at the outset as those experienced during processingdue to excessively smooth surfaces, in particular polymers, which can bein the form of films or sheets.

Examples of such polymers are, in particular, polymer films of naturaland synthetic polymers, such as cellulose, cellulose esters, polyvinylchloride, polystyrene, styrene copolymers with butadiene, polycarbonate,polymers and copolymers of olefins, such as ethylene, propylene,butylene, methylpentene, etc., polysulfone, aliphatic and aromaticpolyesters, polyimides, polyisobutylene, polymethyl methacrylate,polyphenylene sulfide, polyurethane, polyamide, polyaryl ether ketone,polyaryl ether ether ketone, polyaryl ether sulfone, polyamidoimide,polyether imide, etc., (cf. Ullmanns Encyclopadie der TechnischenChemie, Verlag Chemie, Weinheim, 4th revised and extended edition(1976), page 673 et seq.; Encyclopedia of Polymer Science and Eng., Vol.7, John Wiley & Sons (1973), page 73 et seq. ). The production ofself-supporting, oriented or non-oriented films from the stated polymersis carried out by various known technologies, which are likewisedescribed in the literature cited. The term self-supporting film isintended to be understood to include both monofilms consisting of onelayer and multilayer films consisting of a plurality of layers ofidentical or different polymer materials, or laminates containing layersof plastics and of non-plastics, such as, for example, paper or metal.

The coating on the substrate layer is formed by the simultaneous actionof an electric corona discharge in the presence of an aerosol containingfilm-forming agents. The corona discharge triggers and/or promotesreaction mechanisms which result in chemical surface modification of thetreated substrate. It is assumed that, as a result of the coronadischarge, the aerosol or the agents contained therein is or are firmlyanchored to the substrate surface.

For the corona discharge, an alternating current voltage between 5,000and 25,000 V is applied to the high voltage electrodes by means of ahigh voltage generator. The alternating current voltage between the highvoltage electrodes and the earthed counter-electrode should beproportional to the transport velocity at which the substrate to betreated is moved through the corona discharge zone, i.e., at a highertransport velocity the alternating current voltage should be within theupper voltage range while at lower transport velocities a medium to lowvoltage in the range of 5,000 to 18,000 V can be set in order to achievean effect of the same magnitude.

The aerosols can be produced with the aid of known two-materialatomizing nozzles or preferably by means of piezoelectric ultrasonicatomizing systems. Aerosols can be produced using emulsions, preferablydispersions or solutions of organic polymeric binders, which are takenin solvents or dispersants, very particularly preferably in water.

A process for the treatment of the surfaces of plastics substrates bymeans of corona discharge with simultaneous spraying with an aerosol,and an arrangement for carrying out this process, have already beendescribed in every detail in German Patent Application P 37 05 482.1,corresponding to U.S. Ser. No 07/156,572, filed Feb. 17, 1988, now U.S.Pat. No. 4,929,319 assigned to the assignee of the present invention.

Examples of the film-forming polymeric agents are homopolymers orcopolymers based on acrylic acid or methacrylic acid or theirderivatives, ncncrystalline copolyesters or copolyester ethers, whichmay contain alkali metal-containing building blocks, such as, forexample, the sodium salt of 5-sulfoisophthalic acid, in amounts of from1 to 50 mol percent, or mixtures of these. Acrylate-containingcopolymers which are crosslinkable, for example with melamine,polyurethanes, polyolefins or ionomers, which are dispersed in water,are also suitable film-forming agents. Further examples are aqueoussolutions or emulsions of bifunctional fluoropolyethers, in particularbifunctional derivatives, such as diols, dicarboxylic acids anddiesters. Further lubricants which are added include monocarboxylicesters of glycerol with long-chain fatty acids, such as glycerolmonostearate, and organosiloxanes, such as polydiorganosiloxanes. Viathe choice of the film-forming agents, it is possible to impart to thesubstrate not only the sliding properties desired according to theinvention but also further valuable surface properties, such as improvedadhesion to applied materials, sealability, etc.

The amount of film-forming agent to be applied can be varied within widelimits and is preferably tailored to the particular relevantapplication.

In the process according to the invention, the film-forming agents arefirst dispersed, emulsified or dissolved in a solvent or dispersant, anaqueous solvent or dispersant preferably being used, and are thensprayed together with a carrier gas stream into the corona dischargespace as an aerosol. The dispersions or emulsions generally have asolids content or droplet content of from 0.5 to 20 percent by weight,based on the total weight of the dispersion or emulsion, while thesolutions have concentrations in the range from 1 to 30 percent byweight. It may be advantageous to increase the temperature of thecarrier liquid to improve the dispersibilty or emulsifiability.

In the simplest case, air is used as the carrier gas for the aerosol,but is is also possible to use nitrogen or another inert gas, forexample a noble gas. On the other hand, particularly advantageovsanchoring effects can also be achieved with reactive gases, such as, forexample, with halogens, carbon dioxide, ammonia or formaldehyde, mixedwith inert gases if necessary.

In the process according to the invention, the coating can be applied tothe substrate layer off-line after the substrate has been produced. Thisapplies particularly when sheet-like substrate layers are to be coated.However, the coating can also be applied just as well in-line, duringthe process for the production of the substrate layer. It is known that,for example, biaxially oriented films of thermoplastics are produced byextrusion of a melt and subsequent biaxial orientation, followed by heatsetting. In this procedure, the coating can be applied before, betweenor after the individual stretching steps which take place one after theother, or can be applied before or after the heat setting.

Because of the particular application process according to theinvention, the coating has particularly good adhesion to the substratesurface. The layer thickness of the coating usually corresponds to aweight per unit area of the substrate to be coated in the range of only1 to 100 mg/m².

Suitable applications for the sheet-like structures according to theinvention are all those applications where good sliding properties aredesirable. For example, the use of packaging films on high-speedpackaging machines or films for technical applications, such as, forexample, adhesive tapes, magnetic tape films, capacitor dielectrics orsubstrate films for reprographic or photographic applications.

The invention is illustrated in more detail below with reference to FIG.1 which shows an aerosol/corona apparatus for treating various polymerfilms. The apparatus consists of a generator 5, which is connected to acorona discharge apparatus 11. The generator 5 applies a high-frequencyalternating current of high voltage to discharge electrodes 4 of thecorona discharge apparatus 11. The metal core 2 of a roller 10 having acircumferential polymeric coating 3 serves as the groundedcounter-electrode. A self-supporting polymer film web 1 to be coated istransported over the roller 10. A housing 6 encloses the dischargeelectrodes 4 of the corona discharge apparatus 11 and is connected via aline 7, for example a pipe of flexible tube, to an atomizing apparatus8, in which an aqueous liquid is atomized to give a suspendible aerosolby means of a two-material atomizing nozzle or a piezoelectricultrasonic system. A blower 9 is connected to the atomizing apparatusand conveys the carrier gas (air) for the aerosol through the atomizingapparatus into the corona discharge apparatus 11. The line 7 isconnected to a pipe socket 13 of the atomizing apparatus 8 and to a pipesocket 12 of the housing 6.

The examples which follow are intended to give a clearer description ofthe invention and the reproducible advantages which can be achieved withthe invention.

EXPERIMENTAL CONDITIONS

Various polymer films were treated by means of an aerosol/coronaapparatus corresponding to the accompanying figure. Unless statedotherwise, the transport velocity of the self-supporting polymer filmswas 20 m/min. The corona power in each case was 1,000 W (correspondingto a setting of 5,500 V). The surface roughness was determined bymeasurements according to DIN 4768. The slip properties of the film aredetermined by measurements according to DIN 53,375. The friction weremeasured at five measuring points distributed over the entire filmwidth.

To form a substrate layer, a pigment-free polyester raw material havinga saponification value of 810 was melted into a film and formed by meansof a slot die, and quenched on a cooled roller polished to a high glossto give an amorphous film. The amorphous preliminary film was stretchedlongitudinally at 95° C. and then stretched in the transverse directionat 110° C. The stretching ratio per unit area being 13. The resulting 15micron thick film was then heat-set at 230° C. (film temperature). Thefilm was then coated using the apparatus described.

EXAMPLE 1

The film was treated during corona treatment, according to theinvention, with a dispersion containing a crosslinkable acrylate andhaving a solids content of 4 percent by weight. The exact composition ofthe acrylate was as follows: 52 mol percent of methyl methacrylate, 43mol percent of ethyl acrylate and 5 mol percent of methacrylamide. Theacrylate was mixed in a ratio of 4:1 with a crosslinking agent based onan 80/20 mixture of melamine/formaldehyde. The coating was applied at arate of about 20 mg per square meter of film substrate. FIG. 2 shows ascanning electron micrograph of the surface of the resulting film under300 times magnification. This micrograph clearly shows how a textureresembling lunar craters was formed, in which large and small, more orless closed structures are distributed in a purely random arrangementover the entire surface of the substrate layer, corresponding to impactin each case with smaller or larger aerosol droplets. The coefficient ofstatic and sliding friction (DIN 53,375) of the film was 33. The filmhaze was 1 percent, the haze being determined according toASTM-D-1003-61, Method A.

EXAMPLE 2

Example 1 was repeated, except that the aerosol was formed with the aidof an aqueous dispersion containing a copolyester consisting of 65 molpercent isophthalic acid units and 5-sulfoisophthalic acid units (35 molpercent). FIG. 2 shows a scanning electron micrograph of the filmsurface produced according to Example 2, under 300 times magnification.A structure resembling a crater landscape was likewise recognizable,but, owing to the lack of crosslinking, the individual structuralelements do not contrast in color with the background. The film hazeaccording to Example 2 was 0.5 percent.

EXAMPLE 3

Example 1 was repeated by using an aqueous solution (concentration: 4percent by weight) of polyvinyl alcohol. With the aid of the scanningelectron microscopic method of investigation, no textures weredetectable on the film surface thus produced.

EXAMPLE 4

To form a substrate, a polyethylene terephthalate raw material whichcontains 1,000 ppm of SiO, as an antiblocking agent was melted in anextruder, formed with the aid of a slot die into a film and quenched togive an amorphous preliminary film. The preliminary film was thenstretched longitudinally at 95° C. and stretched in the transversedirection at 110° C. The stretching ratio per unit area was maintainedat about 13. The resulting 50 micron thick film was then heat-set at230° C. The heat-set film was then further treated by four processvariants. The heat-set film was subjected to a corona treatment and asimultaneous aerosol treatment corresponding to the process according tothe invention. The aerosol was produced from a 0.12 percent by weightaqueous solution of a bifunctional polyfluoropolyether of the formula

    HOH.sub.2 C--R.sub.F --CH.sub.2 OH,

where R,=polyfluoropolyether. The sliding friction according to DIN53,375 measured at five points over the film width, was very uniform andwas 0.14 to 0.15. The amount of coating applied to the film was about 1mg per square meter of film.

The film obtained exhibited strong anchoring of the slip-improvingcoating to the substrate. The stability during sustained use or theanchoring of the coating under mechanical load was tested by wiping thefilm surface ten times with a cotton wool ball under contact pressure inthe range from 1 to 5 kPa. The new sliding friction then measured showsno change compared with the coefficients of sliding friction initiallymeasured.

COMPARATIVE EXAMPLE 1

The heat-set film was subjected to neither a corona treatment nor anaerosol treatment. The sliding friction was about twice as high as inthe above mentioned example and was in the range from 0.28 to 0.32. Thefluctuation in the sliding friction, measured over the film width, wassmall and a good uniformity of the friction over the film width waspresent.

COMPARATIVE EXAMPLE 2

The heat-set film was subjected to a corona treatment without an aerosoltreatment. The sliding friction was about 2. 6 to 2 .7 times the valueof the example according to the invention and was in the range from 0.37to 0.40. The fluctuation in the sliding friction, measured over the filmwidth, was small, i.e., the uniformity of the friction over the filmwidth was good.

COMPARATIVE EXAMPLE 3

The heat-set film was subjected to an aerosol treatment without a coronatreatment. The coating employed was the same as that in Example 1. Theamount of coating applied to the film was also the same as in Example 1.Examples 1 and 2. It was about 1.6 to 2.1 times the value of the exampleaccording to the invention and was in the range from 0.22 to 0.29.

The fluctuation in the sliding friction, measured over the film width,was large so that the uniformity over the film width must be regarded aspoor.

The results of the measurements of the sliding friction of the exampleaccording to the invention and of Comparative Examples 1 to 3 aresummarized in the Table below.

                  TABLE 1                                                         ______________________________________                                        Sliding friction according to DIN 53,375                                                              Uniformity of the                                     measured at five measuring points over the                                                            fiction over the                                      film width              film width                                            ______________________________________                                        Measuring                                                                              1      2      3    4    5                                            Points                                                                        Example  0.14   0.15   0.14 0.15 0.15 Very good                               Comparative                                                                            0.30   0.29   0.31 0.28 0.29 Good                                    Example 1                                                                     Comparative                                                                            0.38   0.40   0.37 0.39 0.38 Good                                    Example 2                                                                     Comparative                                                                            0.22   0.28   0.23 0.29 0.26 Poor                                    Example 3                                                                     ______________________________________                                    

That which is claimed is:
 1. A process for the production of a filmstructure, comprising treating at least one surface of a substrate layerby means of a corona discharge operated at an alternating currentvoltage between 5,000 and 25,000 V, said corona discharge occurringbetween a high voltage electrode and a counter electrode; andintroducing an aerosol containing a polymeric film-forming agent with acarrier gas into the corona discharge.
 2. A process as claimed in claim1, wherein treating said substrate includes moving said substratethrough the corona discharge zone at a transport velocity proportionalto the level of the alternating current voltage at which the coronadischarge is operated.
 3. The process as claimed in claim 1, wherein theaerosol is introduced into the corona discharge zone with the aid oftwo-material atomizing nozzles or piezoelectric ultrasonic atomizingsystems.
 4. The processes claimed in claim 1, wherein said polymericfilm-forming agent is dispersed or emulsified in a solvent or dispersantand said dispersion or emulsion has a content of polymeric agent of 0.1to 20% by weight, based on the total weight of the dispersion oremulsion.
 5. The process as claimed in claim 1, wherein the carrier gasused is a gas, selected from the group consisting of air, nitrogen or anoble gas.
 6. A process as claimed in claim 1, wherein the carrier gasused is a gas, selected from the group consisting of halogen, carbondioxide, ammonia, or formaldehyde, or a mixture of said gases with gasesselected from the group consisting of air, nitrogen, or a noble gas. 7.A process for forming a polymeric film structure with improved slipcharacteristics and reduced friction properties, comprising: forming apolymeric film structure; corona treating at least one surface of saidstructure; and coating said structure simultaneously during said coronatreatment by means of an aerosol, said aerosol comprising a carrier gasand a dispersant, said dispersant comprising a solvent and afilm-forming agent, said coating applied at a rate of from 1 to 100 mgper square meter of said structure.